It is no surprise that decomposing food can smell bad. How a composting facility is designed and run, as well as what kinds of things it processes–and even the weather and the seasons–contribute to whether nearby communities experience odor. Specifically:

The Pile…

What Happens…


It Should Be…

Is too big


Too much waste won’t decompose properly

Proportional to the space and content

Is not being turned enough

Fermentation—frequent mixing gets air flowing, which prevents this

16–18% oxygen

Is too wet

Fermentation or anaerobic conditions

>50% water

Has an unbalanced carbon-to-nitrogen ratio

Pile degrades too slowly; seasons and the amount of what gets sent for composting contribute to the imbalance (e.g., lots of grass in the spring)

C : N = 25:1–30:1

Has a low pH

It’s a bad environment for good microbes—ones that feed on waste and help it decompose—and a good environment for bad microbes—ones that contribute to the odor; especially a problem with newly dumped feedstocks

Between 6.5–8.5

Is ground up too much

Not enough air gets through the piles and it degrades too slowly

Between "really small" to <1, but the sweet spot is a mix of sizes

Has not sat long enough

Piles haven’t had enough time to decompose—this is especially problematic when the pH is too low

Depends on a variety of factors

The feedstocks themselves (i.e., what gets sent for composting) are a major contributor to odor–and they vary from load to load and season to season. Recently delivered feedstocks are often the main source of odor at composting sites.

To help keep odor low, facilities monitor the piles’ temperature, oxygen, moisture, pH, and nitrogen- and carbon levels. Facilities also must accommodate seasonal changes—temperature, wind, and rain, as well as what gets sent for composting.

The Odor Wheel

As organic waste decomposes, chemicals are emitted. Each chemical has a unique “odor signature,” meaning that it smells a certain way—like that “rotten egg” smell we all know.

The sense of smell is a survival tool that alerts us to danger. It is processed by the part of the brain that handles memory and emotions. Although we experience odor as extremely unpleasant, it isn't necessarily harmful to your health. For example, researchers have found that people can smell the odor signature of a compost component long before that chemical could pose a health threat. However, just because it's not technically harmful doesn't mean your quality of life isn't compromised.

To address this, scientists developed the Odor Wheel. The Odor Wheel helps people identify specifically what they might be smelling when downwind from a composting facility—various odors could indicate that something in the process might be out of balance.

Download a copy of the Odor Wheel here.


Preventing Odor

In a report produced for the California Integrated Waste Management Board, researchers outlined practices that industrial composting facilities can use to mitigate odor:

General Facility-wide Practices

  • Frequently monitor material for pH, moisture levels, aeration, and carbon-to-nitrogen ratio.
  • Collect feedstocks more frequently from the populace–feedstocks that sit for long periods before pick up are more likely to be odorous.
  • Mist material with water or an odor-neutralizing product during every stage of the process.
  • Schedule moving, turning, and grinding of material when weather conditions are favorable–especially wind.
  • Do not allow water to pool anywhere on site–standing water and leachate from compost create strong odor.
  • Monitor after big rains.


  • Upon receiving food waste, immediately incorporate into a pile of already composting woody material.
  • Upon receiving food wasteb place into a basin and mix with a bulking agent (wood chips, straw).
  • Enclose the receiving area and add a negative aeration system.
  • Reject loads that are noticeably more odorous.
  • Do not allow loads to sit onsite unprocessed.
  • Set limits for total amount of input material–this will help prevent facilities exceeding their handling capacity.
  • If unprocessed material needs to sit onsite, place it in piles with low surface-to-volume ratio to increases airflow and decrease odor formation.


  • Increase facility's grinding, screening, and processing capacities to be able to handle its level of inputs.
  • Grind materials in smaller amounts or mix especially large quantities of food/green waste with woody material.
  • Enclose grinding and mixing area so that it can be done at any time or weather condition.


  • Limit pile and windrow size to 10 feet high or less.
  • Use a pseudo biofilter/blanket layer of finished compost or oversized woody pieces.
  • Water the blanket layer so odors may absorb onto woody particles.
  • If pH is low, add lime or wood ash to piles.
  • With noticeably odorous loads, immediately mix into already active windrows with established microbes that can treat odorous compounds.
  • Keep moisture levels in windrows between 40% and 60%.
  • Ensure aisles between windrows are clean and free of any decomposing material.
  • Make compost piles smaller, more porous and mixed with more carbon so they compost cooler and slower, and the bugs have what they need to work.
  • Run slow blowers to allow the compost pile to stay wetter and cooler, and treat odor, rather than relying on biofilters to treat air emissions.
  • Place a blanket of finished compost over the active piles.
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